1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser and a surface emitting semiconductor laser array which are used as a light source for optical disks, laser printers and laser displays.
2. Description of the Related Art
As the method of monitoring the light output of a surface emitting laser, there is proposed a method of monitoring light split by a beam splitter provided at the light emission window of a laser element package with light receiving elements provided in the package (Japanese Published Unexamined Patent Application No. Hei 8-330661). However, in this method, the laser package has a special structure and a general-purpose product cannot be used, thereby increasing costs. In addition, the beam splitter and the light receiving elements having a relatively large area are used, thereby boosting costs. In this method, when a surface emitting laser generates multiple beams, beam outputs cannot be monitored independently and the whole size of the laser cannot be made compact. Therefore, a technology for forming a photodetector on a surface emitting laser monolithically is desired.
As the method of forming a photodetector monolithically, there are proposed a method of forming light receiving elements on the structure layer of a surface emitting laser and a method of forming light receiving elements in parallel next to a surface emitting laser.
The method of forming light receiving elements on the structure layer of a surface emitting laser is first proposed by U.S. Pat. No. 5,136,603. This method is to monitor the light output of a surface emitting laser by forming an i type layer and an n type layer on a p type DBR (distributed Bragg reflector) mirror layer on the front side of the laser to form a columnar pin photodetector and causing the laser to emit light through the pin photodetector. However, in this structure, the pin photodetector destroys the cyclic structure of the DBR mirror, thereby reducing reflectance. Since an p type electrode formed in the p type DBR is shaped like a ring surrounding the columnar pin photodetector, a current injected from this electrode concentrates on the peripheral portion of the light emitting part of an active layer, whereby an output light intensity distribution hardly becomes a unimodal convex but has a high-order horizontal mode. This causes such a problem that the laser cannot be used as a light source for optical disks and laser printers.
U.S. Pat. No. 5,757,837 improves the above invention of U.S. Pat. No. 5,136,603 and proposes a structure that an intra-cavity type pin photodetector is embedded in a DBR mirror on the front side of an intra-cavity type surface emitting laser. Since this laser has an intra-cavity structure, as shown in FIG. 15A, a laser drive electrode (p type electrode 50) on the front side of a substrate is formed on an upper spacer layer 16 or at an intermediate position of a DBR mirror 17. An n type electrode 21 for driving a laser is formed on the rear side of the substrate.
The pin photodetector is constituted such that a quantum well which is an optical absorber layer is made from 80 .lambda.- Ga.sub.0.8 In.sub.0.2 As, sandwiched between GaAs spacer layers and further sandwiched between laminatestructured DBR mirrors having a thickness of .lambda./4. This photodetector shares a p type electrode 50 for driving a laser as the p type electrode of the photodetector, and a reverse bias is applied by a power source 54 to a space between an n type electrode 52 for a photodetector formed on the top surface and the p type electrode 50 to monitor the quantity of laser light with an ammeter 56.
In this patent, the structure of the surface emitting laser is formed of an intra-cavity type to prevent a laser drive current from running into the photodetector. However, this intra-cavity type surface emitting laser having the photodetector formed monolithically has the following problems.
Since the p type electrode 50 is formed outside thepost-shaped upperDBRmirror in the intra-cavity type surface emitting laser, the hole of the ring of the p type electrode 50 becomes large. Therefore, a current injected into the active layer from the p type electrode 50 concentrates upon the peripheral portion of the light emitting part. Accordingly, the horizontal mode is apt to become a high-order mode and a unimodal light intensity distribution is hardly obtained. To prevent these to some degree, a current strangulation structure for reducing the diameter of a current path is generally formed in a spacer layer or a DBR mirror near the spacer layer. In spite of this, a current concentrates upon a portion around the current path and a light emitting part concentrates upon that portion. A current injected into the active layer from the p type electrode moves in a horizontal direction, resulting in large electric resistance. Particularly when the p type electrode is formed on the spacer layer, a current moves horizontally in the narrow spacer layer, resulting in a large resistance value.
In this intra-cavity type surface emitting laser, since the pin structure of the photodetector is a quantum well structure, photocarriers generated in the quantum well by optical absorption are kept in the quantum well and hardly taken out to the outside. Therefore, the current value greatly depends on a reverse bias voltage applied to the photodetector. Therefore, to monitor laser light properly, voltage applied to the p type electrode 50 and the n type electrode 52 of the photodetector must be maintained at a fixed level. That is, as shown in FIG. 15A, while the potential of the p type electrode 50 shared by the laser and the photodetector is grounded and a fixed reverse bias voltage is applied to the n type electrode 52 of the photodetector, the laser is driven by operating the potential of the n type electrode 21 for driving a laser. FIG. 3B shows an equivalent circuit for a surface emitting laser having this photodetector. In the case of a single-beam laser, this drive method has no problem. However, in the case of a multi-beam laser, since an n type DBR mirror is generally electrically connected, all the lasers can be driven simultaneously and hence, this drive method cannot be employed. To eliminate this problem, as shown in FIG. 15B, lasers which are grown on a semi-insulating substrate 58 must be isolated from one another by etching. However, this case involves serious problems that the production process becomes complicated and that surface emitting lasers cannot be arranged at a small pitch.
U.S. Pat. No. 5,748,661 proposes a method of forming light receiving elements in parallel next to a surface emitting laser. In this patent, a cylindrical photodetector is formed to surround a columnar surface emitting laser so as to detect natural emission light leaked from the surface emitting laser. Since natural emission light not contributing to laser oscillation and not laser oscillation light is detected in this method, the quantity of light detected increases as the amount of an injection current becomes larger until a laser oscillates. However, once the laser oscillates, the quantity of natural emission light rarely increases even when the amount of an injection current grows. Therefore, an increase in the current value for monitoring the quantity of light is extremely small though the quantity of laser light increases, thereby making it impossible to carry out monitoring properly. Further, since the photodetector has the same pin structure as the surface emitting laser, carriers generated in the active layer which is an i layer are kept in a potential well and hardly taken out to the outside. As the intensity of natural emission light is low, the value of current generated in the photodetector by the light is small, thereby making difficult the detection of the natural emission light.
As described above, in the method of forming a pin photodetector structure in the laminate structure of an intra-cavity type surface emitting laser monolithically as disclosed by U.S. Pat. No. 5,757,837, the insertion position of the photodetector is farther away from an active layer than a pair of laser drive electrodes for flowing a current in the laser active layer. Therefore, the surface emitting laser has an intra-cavity structure and the laser drive electrodes are shaped like a ring surrounding the photodetector. As a result, a laser drive current concentrates upon the peripheral portion of the light emitting part of the active layer and an emission light intensity distribution is apt to become a multi-mode and not a single-mode. At the same time, the current path is formed in a horizontal direction or an oblique direction with the result that its electric resistance becomes high or its production process becomes complicated, thereby reducing laser performance. Since a conventional pin photodetector inserted into the laminate structure of a DBR mirror has a double-hetero structure such as a quantum well structure, a current generated by optical absorption is hardly taken out to the outside. At the same time, since the value of current taken out to the outside from the photodetector greatly depends on a reverse bias voltage applied to the photodetector, the reverse bias voltage must be maintained at a fixed level. Under the conditions that the p type electrode for a laser (which is also a p type electrode for the photodetector) is grounded as a common electrode and the potential of the n type electrode for a photodetector is fixed, the laser must be driven with the n type electrode on the rear side of a substrate for driving a laser. Therefore, in the case of a multibeam laser, after laser crystals are grown on a semi-insulating substrate, each laser element must be isolated by etching, thereby making its production process and electrode wiring structure complicated. Therefore, a laser array having a narrow pitch cannot be manufactured.
The present invention has been made in view of the above problems of the prior art and provides a surface emitting semiconductor laser having a photodetector capable of monitoring the light output of the laser easily at a high speed without deteriorating the performance of the surface emitting laser. The present invention further provides a surface emitting semiconductor laser array having a photodetector capable of monitoring the light output of each laser with a simple structure.